Membrane channels in the sarcoplasmic reticulum of striated skeletal muscle were shown to be selective for calcium using a patch clamp technique, and to have a voltage dependent conductance. Spinal cord neurons cocultured with muscle cells induce a decrease in the incidence of slow hyperpolarizing after potentials following an overshoot of the action potential. A soluble factor released by these neurons, and causing this effect, was isolated, with a molecular weight of less than 4000 daltons. Dorsal root ganglia neurons from tisomy 16 mice, a model for trisomy 21 (Down Syndrom) in humans, were maintained in tissue culture, and show to have different electrical properties than control neurons.